1. Field of the Invention
This invention relates to an improved method of fabricating stiff and strong lightweight structures, and more particularly, to an improved method for the fabrication of silicon carbide (SiC) and/or silicon (Si) lightweight structures by the utilization of conventional vapor deposition techniques. Such lightweight structures have utility in a variety of diverse applications including back-up structures for optical components, as structural components for automobile, aerospace and space applications, and as lightweight furniture parts for space.
2. Description of the Prior Art
In the field of optics, light detection and ranging (LIDAR) has come to be recognized as an important diagnostic tool for remote measurement of a diversity of atmospheric parameters such as minor species concentrations, pressure, temperature, and water vapor profiles, aerosol cloud distributions, and wind fields. LIDAR techniques such as measurement of back scattered signals, differential absorption, and Doppler shifts have been used to obtain information about the earth's atmosphere.
The performance of a LIDAR system depends upon the optical configuration of its receiving telescope. Often, due to space limitations such as in a shuttle borne LIDAR system, the length of the telescope is fixed. Therefore, the optical designer must select a particular shape and optics speed of the mirrors to maximize the throughput of the telescope. The most critical element in the receiving telescope is the primary mirror because of its size, weight, fabrication cost, and thermal exposure to the outside world. Since the received signal is directly proportional to the area of the primary mirror, it is important to use as large a primary mirror as feasible to obtain reasonable signal levels for accurate measurement. This is particularly true when a space-borne LIDAR system is used to measure wind profiles in the troposphere on a global basis.
The conventional techniques employed in the prior art for fabricating large (.gtoreq.1.0 meter diameter) mirrors are quite slow and time consuming. Several months to years are required to fabricate a large mirror from ultra low expansion silica glass or Zerodur, a product commercially available from Schott Glass Technologies, Inc., 400 York Avenue, Duryea, Pa. 18642. Since a number of space-based LIDAR systems are planned for the future, considerable attention is currently being given to the development of techniques for the rapid and economic production of large, high performance mirrors.
Thus, a spin casting technique has been proposed to fabricate 1.2 meter and 3.5 meter diameter glass mirror blanks containing lightweight honeycomb cells. Although this technique is relatively faster than the conventional mirror fabrication methods and produces lightweight mirrors, the weight of these mirrors is still an order of magnitude more than permissible for many space applications. Further, the spin-casting technique is unsuitable for fabricating large mirrors of advanced ceramics such as SiC, titanium diboride (TiB.sub.2), and boron carbide (B.sub.4 C) that have high melting points. These latter materials have properties superior to those of glass for large lightweight optics.
Other techniques involving the casting of fiber reinforced composites containing epoxy and plastics and the stretching of membranes over appropriate substrates are also currently under investigation.
Still another technique for making stiff lightweight structures is disclosed in U.S. Pat. No. 4,716,064 granted to Robert A. Holzl et al. on Dec. 29, 1987. The Holzl et al. patent emphasizes a requirement for two parallel separated surface defining members that are connected by stiffeners. Fabrication starts with a solid graphite disc which defines the outer envelope to the part to be produced. Then, by a series of drillings of bores or holes in the graphite disc, the use of plugs and multiple coatings of a chemically vapor deposited material possessing a high stiffness to weight ratio, the part is constructed. A disadvantage of this fabrication procedure is that it is time consuming, complex and costly. Moreover, the many steps of drilling, plugging, and multiple coating involved inherently limit the ability to control figure stability. This impairs the value of the process where extreme figure stability retention is of importance, as in high performance mirrors. Additionally, the Holzl et al. technique is limited to relatively thin structures because of the difficulty of obtaining uniform coatings in the passages between the spaced parallel surface defining members.
Thus, there is a need and a demand for improvement in the methods of fabrication of stiff and strong lightweight structures to the end of achieving extreme figure stability retention as well as an amenability to being scaled up in size while at the same time enabling simplification in the fabrication procedure and reduction in the time required for and the cost of such procedure.